1. Field of the Invention
This invention relates to a hammer drill for drilling through rock formations, having an improved hammer piston and valve arrangement which allows more effective penetration of hard rocks.
2. Prior Art
It is known to provide a hammer drill with a piston operated by liquid, usually water, supplied to the top of a drill string, and which is capable of providing downwardly directed impacts on an anvil forming part of, or being connectible to, a drill bit. Valves are provided to allow continuous cycling of the piston to give repeated impacts.
In such prior hydraulic hammers, when the piston is abruptly stopped by hitting the anvil, the liquid pressure suddenly increases to a peak, putting strain on the drilling string. It has been proposed in accordance with International Patent Application No, WO 94/23171 (Mclnnes) to reduce these strains by having two pistons which reciprocate 180.degree. out of phase with each other. This doubles the frequency of the impacts and reduces the pressure peaks during drilling, but results in impacts which are not as forceful as would be obtained with a single piston.